A new kind of potential respiratory component has been found in E. coli membranes. It is analogous to cytochrome oxidase in that its midpoint potential and multiplicity for electron transfer are very similar. It can be physically separated from cytochrome oxidase by extraction with deoxycholate. Its optical and electronparamagnetic properties identify it as a high potential iron sulfur protein or HIPIP. It appears to require copper and lipid for its proper function which involves the oxidative generation of free acid. All indications point to phosphate buffer as the source of the protons.